Electrostatics is a branch of physics that deals with the phenomena of electrical charge attraction and repulsion. Electrostatic systems have been used to improve coating efficiency, for example, of paints applied to metallic products such as refrigerators, desks and automobiles.
The use of electrostatic delivery systems to apply various powdered coating compositions on electrically conductive objects is described in German OLS Patent Application 3 014 226, Weigel, published Oct. 15, 1981 (powder lacquer mixed with a nonpigmented powder lacquer to form a "solid mixture" is electrostatically applied to electrically conductive objects to increase film properties against external influences) and Japanese Patent Publication 48-9472, Sakai et al., publication date Mar. 24, 1973 (powder composition containing polyamide mixed with N-substituted sulfonamide for electrostatic coating of metal objects).
In electrostatic powder coating, an electrostatic charge is imparted to the powder, which is then delivered to the object to be coated. The object is held at electrically ground potential and the powder coats the object because of the attractive force between the grounded object and the charged powder. In an electrostatic powder spray gun, the powder is pushed through a central passageway by pressurized gas. The central passageway contains electrodes which ionize the air surrounding them. The powder traveling by the electrodes acquires these ionic charges. The charged powder is then emitted from the gun, usually in a conical spray pattern, in the area of the object to be coated.
U.S Pat. No. 4,780,331, Cobbs, Jr. et al, issued Oct. 25, 1988, describes a particular method and apparatus for charging powder particles by electrostatic induction. See also U.S. Pat. No. 4,380,320, Hollstein et al., issued Apr. 19, 1983 (nozzle assembly for an electrostatic spray gun).
Electrostatics is in use for industrial finishing applications. It is used for coating relatively large objects one at a time, such as hubcaps, metal desks, and cans. Examples of commonly used powder coating materials are epoxy, polyester, and porcelain. The art describing application of powder to objects generally speaks of grounded, usually metallic, larger, single objects, rather than individual, small, ungrounded, nonmetallic particles.
The use of a finely divided powder coating on granular detergent compositions to improve detergent flowability and decrease lumping and caking is known. See U.S. Pat. No. 3,868,336, Mazzola et al., issued Feb. 25, 1975 (finely divided flow-promoting agent externally applied to a heat-dried detergent composition to inhibit caking); Japanese Patent 86-333245 (granular detergent composition prepared by (a) kneading certain detergent components, (b) comminuting this solid mixture, and (c) covering the comminuted mixture with water-insoluble, finely divided powder of a primary particle size not above 10 microns); and Japanese Patent 62-45696 (granular detergent composition which is a pulverization product of a detergent ingredient mixture coated with water-insoluble fine powder of an average grain size less than 10 microns and containing other specific detergent ingredients).
U.S. Pat. No. 4,027,366, Millar et al., issued June 7, 1977, discloses a process for electrostatically applying a multi-layered coating on a substrate in one operation. A mixture of powders is used as the coating composition. At least one of the powders is a film-forming non-conductive polymer. The powdered composition is applied to a conductive substrate which has a neutral charge or a charge opposite from that of the coating composition powder particles. According to Millar et al, the powders then stratify into layers and adhere to the substrate because of contact or static electrification for a reasonable length of time and until at least one of the powders can be cured or fused to form the final coating.
Aluminosilicates, or zeolites, are of particular interest as finely divided powders for detergent granule coating. A series of Wixon patents describe heavy duty laundry detergents comprised of particles having nonionic detergent in the interior and on the surface thereof to which is adhered a coating of smaller particles of ion-exchanging zeolite. Various such heavy duty laundry detergents and methods for making them are disclosed. See Wixon's U.S. Pat. Nos. 4,248,911, issued Feb. 3, 1981; 4,260,651, issued Apr. 7, 1981; 4,339,335, issued Jul. 13, 1982; 4,347,152, issued Aug. 31, 1982; 4,411,809, issued Oct. 25, 1983; 4,666,738, issued May 19, 1987; and 4,666,740, issued May 19, 1987.
In summary, although coating detergent granules with finely divided powder is known, the instant process allows for efficient coating (reduced dust, less powder recycle, less powder required) and better results (uniform distribution over the granules). The use of electrostatics for coating small, numerous, ungrounded, nonmetallic particles, like detergent granules, with finely divided powder is not known.